Methacrylate resin-based compositions have been used in dentistry for decades. However, dental materials based on (meth)acrylate resins, which are chemically cured, usually have wet surfaces due to the inhibition of the free-radical polymerization reaction at the surface by molecular oxygen. The oxygen inhibition is even more apparent as the rate of polymerization of the material decreases. This presents a potential problem with the restoration of a tooth. For example, when the oxygen inhibition effect takes place in an interface, such as at the tooth surface of a root canal, unsatisfactory sealing may result.
Alternatively, compositions cured by epoxy ring-opening polymerization have been used to seal root canals. For example, products, such as AH 26® Silver Free and AH PLUS JET® by Dentsply, are commercially-available, two-part epoxy-amine compositions. While compositions cured by epoxy ring-opening polymerization are not susceptible to the oxygen inhibition effect, as described above, two drawbacks to these conventional epoxy and amine two-component systems are slower reaction times at room temperature, and lack of adhesion to a tooth surface. Additionally, the slow curing time inhibits the material from possessing instant mechanical properties because the setting time of the material can take hours, if not days.
Compositions cured by epoxy ring-opening polymerization may also be initiated through a cationic polymerization of the epoxy resin using a starter, such as a Lewis acid or a strong Brønsted acid. For example, Lewis acids, such as BF3.Et2O, BF3.THF, AlCl3, FeCl3 and the like, where Et2O is diethyl ether and THF is tetrahydrofuran, may initiate the cationic ring-opening polymerization under ambient conditions. Similarly, strong Brønsted acids, such as HBF4, HB(C6F5)4, HPF6, HAsF6 or HSbF6 may initiate the cationic ring-opening polymerization immediately after mixing with the epoxy.
Another type of initiator for the cationic ring-opening polymerization is a latent starter, which upon contact with the epoxy does not itself initiate the polymerization, but starts the polymerization upon contact with an agent that transforms the starter into a form that is able to initiate the polymerization. For example, halonium salts of the general formula Hal-(Ar)2+An−, wherein Hal is a halogen, Ar is an aryl group, and An is an anion, when transformed can liberate H+ as extremely strong acid H+An−. An exemplary latent starter is diaryliodonium compounds, such as diphenyliodonium tetrafluoroborate, diphenyliodonium hexafluorophosphate and the like. Generally, Cu(I) salts and a reducing agent, such as ascorbic acid, are used to transform the halonium salt.
Additionally, the use of light-initiated cationic ring-opening polymerization of epoxy resins in dentistry has grown in recent years, largely due to the lower shrinkage of the resulting composite as compared to the free radical polymerization of (meth)acrylates. But these epoxy compositions cured by the methods described above can still present poor adhesion to a tooth surface.
Therefore, what is needed is a composition that possesses favorable curing properties and improved adhesion.